The invention relates to methods and apparatus for sputter deposition of material into high aspect ratio, reentry shaped contact holes in semiconductor substrates. More specifically, the invention relates to conformally covering the walls of such holes with the deposited material, or filling the holes with the material.
Semiconductor devices are typically multilayered structures fabricated on semiconductor wafers one layer at a time. At least one of the layers of the multilayered structure is a metalization layer which is patterned to produce conductive pathways or wires that electrically interconnect the devices that have been formed on the wafer. The metalization is usually deposited onto a passivation layer (e.g., SiO.sub.2) that previously was deposited over the surface of the wafer to protect and isolate the underlying devices. To enable the metalization layer to make electrical contact to the semiconductor devices under the passivation layer, contact holes or vias are etched through the passivation layer prior to depositing the metal later. When the metal (e.g., Al) is deposited onto the wafer, it fills the contact holes and makes contact with the underlying devices.
It is generally desirable for contact holes to have vertical sidewalls. This is particularly true for the high aspect ratio contact holes (i.e., high length-to-width ratio), which now are commonly used to fabricate semiconductor devices with dimensions in the submicron range. However, vertical sidewalls are not always possible because the plasma etch processes that commonly are used to form the contact holes are difficult to control. If the plasma etch process is not stable, as often can be the case, the layer being etched can be "undercut" so as to produce "re-entry shaped" holes (see FIG. 1). In a "reentry shaped" hole 2, the walls 4 are not vertical as would be ideal; rather, the hole progressively increases in width from the top (i.e., the opening) to the bottom of the hole.
A reentry shaped hole is considerably more difficult to coat or fill with metal 6 than is a contact hole with ideally vertical sidewalls. If a conventional Al sputter deposition is used to coat or fill the hole, the deposited material will tend to build up near the upper edge of the contact holes to form an overhang 8. The overhang will prevent sputtered material from going into and reaching the bottom of the contact hole. For very small, high aspect ratio holes this problem is particularly severe.
One can use a coherent sputter deposition to prevent the formation of the overhang. By coherent deposition, we mean that the sputtered material that reaches the wafer is confined by some mechanism to a narrow angular distribution (e.g., the trajectories of the sputtered material reaching the wafer are tightly distributed about a direction that is normal to the surface of the wafer). A coherent deposition insures that more sputtered material will reach, and deposit on, the bottom of the re-entry shaped contact holes. However, if the angle of the re-entrant shaped side walls is too great, even coherent sputtering often cannot completely cover the walls of the holes or completely fill the holes.
In other words, the commonly used sputter deposition and reflow processes are not well suited for reentry shaped contact holes. Specifically, the presence of re-entry shaped contact holes tends to result in poor to nonexistent electrical contacts to the devices at the bottom of the contact hole. Consequently, a wafer conventionally must be scrapped if the process for etching contact holes inadvertantly produces overly reentrant shaped holes. Thus, a need exists for a deposition process for coating the walls of, or filling, reentrant shaped holes, so that wafers having such holes would not have to be scrapped.